Hearme is no problem for me on the Mac - apart from the fact that the G4s are in the office, and I haven't got the bottle to sit in the office singing to a machine, however sexy it looks!
The machine I lug around with me most of the time is an iBook, which is a great piece of kit apart from the fact that it doesn't have a mike hole (or whatever the hole you plug your mike into is called), and I can't yet be arsed to get the USB dongle that functions as a mike hole.
Unfortunately the G3 PowerBook, which would work wonderfully for Hearme almost anywhere, is on semi-permament loan to a colleague who lost his usual laptop under the wheels of a cycle courier. Berk!
However, another is on its way. One day, then...let this be an awful warning (as any who have heard me sing will testify!) As for music files - there are any number of programs that'll play them, from Apple's own QuickTime, through Audion (about the best for MP3s), Sound Jam (there's both a free and shareware version), and SoundApp, which will let you play and fiddle around with virtually any sound format.
Let's face it - Macs were made for making music; leave PCs to the number crunchers and the anoraks!